Thermoelectric converters, which directly convert heat into electrical energy without moving parts, have found applications where simplicity and high reliability are desired. The efficiency of these devices, however, typically is less than 10%.
One system with great potential for higher efficiencies is the alkali metal thermoelectric converter (AMTEC). Current estimates of potential efficiency of energy conversion range from 20 to 40%. See Terry Cole "Thermoelectric Energy Conversion With Solid Electrolytes", Science. Vol. 221, Num. 4614, Sept. 2, 1983.
A prior art AMTEC (also called sodium heat engine) is shown in FIG. 1. The AMTEC design typically requires a condenser shell within which there is disposed a beta"-aluminum solid electrolyte (BASE) tube. The BASE tube structure is filled with liquid sodium which provides sodium ions and electrons. As currently generally understood, the sodium ions are able to pass through the BASE tube structure but the electrons are not. The electrons instead are carried outside of the tube by way of an external circuit with a load. The sodium ions and electrons recombine in a porous electrode that surrounds the BASE tube that is connected to the external circuit. The electrons return to the electrode through an outer grid that is distributed over the electrode. The recombined sodium in the electrode vaporizes along the outer surface of the electrode and travels to a condenser shell where it condenses and collects under the force of gravity in a condensate pool at the bottom of the shell. A pump takes the liquid sodium in the condensate pool and feeds it back into the BASE tube with additional heat energy added. See, T. K. Hunt, Neill Weber and Terry Cole, " Research on the Sodium Heatengine", SAE/P-78/75, 1978.
Most AMTEC units have typically used a 30 cm.times.1.5 cm (I.D.) BASE tube. The power output of these tubes is small, with a range from 50 to 100 watts at approximately 0.5 volts, depending on unit efficiency. Higher power outputs have been achieved, but only by connecting several low power units or tubes together. In one design that achieves several kilowatts of power, six BASE tubes are arranged in two sets (in electrical parallel) of three (in electrical series) around a single heater well. See, T. K. Hunt, J. V. Lasecki, R. F. Novak, J. R. McBride and J. T. Brockaway, "Sodium Heat Engine/AMTEC System Experiments" 4th Space Nuclear Power Conference, Jan., 1987; Research Progress on the Sodium Heat Engine Phase 4, Final Report, DOE/CE/40651-1, 1986, respectively. The heater well supplies thermal energy by radiation heat transfer to the surrounding BASE tubes. This is somewhat different than the approach suggested earlier in which thermal energy is supplied directly to the inside of the BASE tube, but allows for a simpler design by reducing the number of heat transport paths, supplying energy to several BASE tube through a single heat path (heater well).
There are some inherent disadvantages associated with the above approach to a high power density device in which more than 6 BASE tubes are employed. Each BASE tube, for example, must have a sodium supply tube. In a system with many BASE tubes there will be an extensive system of sodium delivery tubes. In addition, the arrangement of BASE tubes around the heater well is not space or thermally efficient. Each heater well occupies space that could be used for another BASE tube. In a triangular pitch arrangement 33% of the tube space would have to be given to heater wells. Also, the temperature drop between the heater well and the BASE tubes would result in a decrease in thermal efficiency. These disadvantages tend to limit the power density achievable.
In order to achieve high power in a compact, low mass, configuration and minimize the heat supply system, it is desirable to (1) supply thermal energy to the inside of the BASE tubes, thereby eliminating the temperature drop and space occupied by heater wells, and (2) use the maximum possible length of BASE tube to reduce the sodium supply complexity. As the BASE tube is made longer, however, it becomes increasingly difficult to provide sufficient thermal energy or working fluid to the inside of the BASE tube. Moreover, and more importantly, the components that provide the electron flow path, both inside and outside the BASE tube, must become larger to accommodate the increased net electrical current generated from the larger BASE tube surface area. But the components, by becoming larger, are then subjected to substantial ohmic (I.sup.2 R) power losses. At a given design length the cross-section of the sodium pool inside the BASE tube or the electron distribution grid outside the BASE tube required to keep ohmic losses less than the power generated exceeds the available space. These constraints limit even the unit length, power density, and minimum weight that can be achieved.